1. Field of the Invention
The present invention relates to a video signal supplying device, and more particularly to a video signal supplying device for sampling a video signal with a sampling clock signal synchronous with a horizontal synchronizing signal separated from the video signal and supplying the sampled video signal as digital data.
2. Description of the Related Art
One conventional video signal supplying device includes a phase-locked loop (PLL) as shown in FIG. 1 of the accompanying drawings. Specifically, the conventional video signal supplying device shown in FIG. 1 comprises a PLL 101 and an A/D converter 102.
The PLL 101 comprises a feedback loop composed of a phase comparator 103 for comparing the phase of a horizontal synchronizing signal separated from a video signal and the phase of an output signal from a frequency divider 106 (described later) and outputting a phase error signal, a low-pass filter (LPF) 104 for passing low-frequency components of the phase error signal from the phase comparator 103, a voltage-controlled oscillator (VCO) 105 for outputting a frequency signal depending on an input control voltage from the LPF 104, and a frequency divider 106 for frequency-dividing an input signal from the VCO 105 at a given frequency-dividing ratio and outputting a frequency-divided signal.
The A/D converter 102 converts an input analog video signal into a digital video signal based on a sampling clock signal supplied from the PLL 101, and outputs the digital video signal.
Operation of the conventional video signal supplying device will be described below.
An analog video signal is supplied to the A/V converter 102. A horizontal synchronizing signal separated from the video signal is supplied to the phase comparator 103. The phase comparator 103 compares the phase of the horizontal synchronizing signal and the phase of a signal indicative of a horizontal scanning frequency from the frequency divider 106, and generates a phase error signal. After high-frequency components of the phase error signal have been attenuated by the LPF 104, the phase error signal is applied as a control signal to the VCO 105 for thereby controlling the frequency of its output signal.
The output signal from the VCO 105 is applied as sampling pulses to the A/D converter 102, and supplied to the frequency divider 106. The frequency divider 106 frequency-divides the supplied signal at a frequency-dividing ratio of 1/m (m is a natural number of 2 or greater), and the frequency-divided signal is supplied as a comparison signal to the phase comparator 103.
In this manner, the PLL 101 reduces any phase error between the phase of the horizontal synchronizing signal and the phase of the output signal from the frequency divider 106, and the VCO 105 outputs pulses which are synchronized in phase with the horizontal synchronizing signal and have a repetition frequency that is m times greater than the horizontal scanning frequency. In the absence of any horizontal synchronizing signal separated from a video signal, the phase error signal becomes nil, and the VCO 105 oscillates at a free-running oscillation frequency.
Another conventional video signal supplying device includes a PLL having a programmable frequency divider as shown in FIG. 2 of the accompanying drawings. Specifically, the conventional video signal supplying device shown in FIG. 2 comprises a PLL 111 having a programmable frequency divider and an A/D converter 112.
The PLL 111 comprises a phase comparator 113 for being supplied with a horizontal synchronizing signal separated from a video signal as a reference input signal, an LPF 114 for converting a phase error signal (differential signal voltage) based on a phase error or difference detected by the phase comparator 113 into a DC voltage, a VCO 115 for controlling an output oscillation frequency based on the DC voltage, and a programmable frequency divider 116 for frequency-dividing the output oscillation frequency from the VCO 115 and supplying the frequency-divided signal as a comparison signal to the phase comparator 113.
The conventional video signal supplying device shown in FIG. 2 differs from the conventional video signal supplying device shown in FIG. 1 in that the output signal frequency of the VCO 115 can be varied depending on the frequency-dividing ratio of the programmable frequency divider 116.
Operation of the conventional video signal supplying device shown in FIG. 2 will be described below. An error signal generated by the phase comparator 113 depending on the phase difference between a horizontal synchronizing signal and an output signal from the programmable frequency divider 116 is converted into a DC voltage by the LPF 114. The DC voltage from the LPF 114 is applied as a control voltage to the VCO 115, thereby varying the output signal frequency thereof. The output signal from the VCO 115 is applied as sampling pulses to the A/D converter 112 and also supplied to the programmable frequency divider 116. The signal supplied to the programmable frequency divider 116 is frequency-divided into a signal which is supplied as a comparison signal to the phase comparator 113.
In this manner, the PLL 111 reduces any phase error between the phase of the horizontal synchronizing signal and the phase of the output signal from the programmable frequency divider 116, and the VCO 115 outputs pulses which are synchronized in phase with the horizontal synchronizing signal. In the absence of any horizontal synchronizing signal separated from a video signal, the phase error signal becomes nil, and the VCO 115 oscillates at a free-running oscillation frequency.
The programmable frequency divider 116 is a frequency divider whose frequency-dividing ratio is variable by a control signal supplied from an input terminal 117. The output signal frequency from the VCO 115 is a frequency depending on the frequency-dividing ratio of the programmable frequency divider 116. Therefore, the output signal frequency from the VCO 115 can be controlled on the frequency-dividing ratio of the programmable frequency divider 116.
The two conventional video signal supplying devices described above pose problems of their own. Specifically, the conventional video signal supplying device shown in FIG. 1 fail to process various video signals having different effective video intervals because the output oscillation frequency of the VCO 105 is fixed. If the conventional video signal supplying device shown in FIG. 1 sampled various video signals having different effective video intervals, video data outputted from the A/D converter 102 would contain unnecessary data, and would not contain necessary data.
The conventional video signal supplying device shown in FIG. 2 is capable of processing various video signals having different effective video intervals by adjusting the frequency-dividing ratio of the programmable frequency divider 116. However, inasmuch as the frequency-dividing ratio needs to be adjusted outside of the PLL 111, the process of adjusting the frequency-dividing ratio has to be carried out each time sources of video signals are changed.